Split electrode gas cell

ABSTRACT

A system for selectively firing one bistable gas cell in a matrix of bistable gas cells forming a display or data storage system. A voltage is applied across the gas cell which is sufficient to sustain a discharge in the gas cell but insufficient to fire the gas cell. Each gas cell within the matrix has a split anode, and a gas cell is selectively fired by applying a positive voltage pulse to one part of the anode and a negative voltage pulse to the other part of the anode.

United States Patent Cotter 51 Jan. 18, 1972 [s41 SPLIT ELECTRODE GAS CELL 3,509,408 4/1970 1112 ..313/220 x [7 2] Inventor: William L. Cotter, Beverly, Mass. Primary Examiner Roy Lake [73] Assignee: Itek Corporation, Lexington, Mass. A ant xaminer-La ence J. Dahl Attorney-Homer 0. Blair, Robert L. Nathans and William C. [22] Filed: -Oct. 1, 1969 Roch 211 Appl. No.: 862,866

[57] ABSTRACT 52 us. (:1. ..315/l69 12 313/210 313/220 A Sysiem selectively fi'ing bistable a matrix [51] Int. Cl. ..H(i5b 37/00 of bistable gas cells forming a display or dam storage System' 581 Field 6: Search ..3l3/6 109.5 210 no A "wage is aPP'ied the gas which is sumciem 15/169 6 sustain a discharge in the gas cell but insufficient to fire the gas cell. Each gas cell within the matrix has a split anode, and a I 56] References Cited gas cell is selectively fired by applying a positive voltage pulse to one part of the anode and a negative voltage pulse to the UNITED STATES PATENTS Other P of the anode- 3,042,823 7/1962 Willard ..3l5/l69 X 5 Claims, 3 Drawing Figures SPLIT ELECTRODE GAS CELL BACKGROUND OF THE INVENTION The present invention relates generally to display or data storage systems which are comprised of a matrix of gas cells. More particularly, the invention relates to a novel system for selectively firing a particular gas cell in the matrix by applying input signals to only one end of the cell.

Most display systems of the prior art which are comprised of a matrix of gas cells have required input signals on both ends of the gas cell to selectively fire a particular gas cell. In these systems, a particular gas cell in the matrix is selectively fired as follows. The X-line on which the particular gas cell is located, which is attached to the anode of each cell on that X-line, is pulsed with a voltage pulse whose magnitude is one-half the required increase in magnitude to initiate a discharge in the cell. The Y-line on which the selected cell is located, which is attached to the cathode of each gas cell along that Y-line, is negatively pulsed with a voltage pulse whose magnitude is one-half of that increase required to initiate a discharge in the cell. In this manner only the particular cell which lies at the intersection of the selected X- and Y-lines has a sufficient voltage across it to initiate a discharge. This type of prior art selection required both ends of the cell to select any particular cell in the matrix for firing. This system also required a high firing voltage, and both a high sustaining voltage and a high sustaining current.

SUMMARY OF THE INVENTION In accordance with a preferred embodiment, a system is disclosed for firing a selected gas cell within a matrix of gas cells from only one end of the cell. Such a system leaves the other end of the cell available for other purposes within the system, or allows the other end of the cell to be tied to a fixed reference. This would allow a common ground for all of the cathodes, which might take the fonn of a thick plate which would serve to conduct heat away from the firing cells and thereby increase the operating life of the cells.

The disclosed invention also allows the operating voltage of each cell within the matrix to be reduced while ensuring that the cells are not accidentally extinguished. This in turn allows the system to operate at a reduced level of power consumption. Another advantage of the disclosed invention is that the voltage required to initially fire each cell can be reduced. This, in turn, allows the current required to sustain firing across each cell to be reduced. This dual effect contributes to a lower total power consumption by the system.

The disclosed invention also provides a unique system for selectively extinguishing a particular gas cell within the matrix. This system ensures that a particular gas cell is not accidentally extinguished until two concurrent signals are received by the cell.

The advantages of this invention are achieved by splitting an electrode of the gas cell into two electrically separated parts. A particular gas cell within the matrix is selectively turned on by applying a positive pulse to one part of the electrode and a negative pulse to the other part of the electrode. During normal operation of each cell both parts of the electrode are maintained at the same voltage, so each one individually ensures that the cell maintains itself in a discharging condition. To extinguish a particular cell both parts of the electrode concurrently receive a voltage pulse to lower the total voltage across the cell below the sustaining voltage. In this manner, the two parts of the electrode achieve an AND-function to extinguish the cell. The anode is the electrode which is split in the preferred embodiment, but it should be realized that the teachings of this invention are also applicable to the cathode.

Although each cell is disclosed as operating in a DC mode after it is fired, it should be realized that it is also possible to operate each cell in an AC mode after firing.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a display matrix of 16 gas cells. FIG. 2 illustrates one gas cell within the matrix of gas cells.

FIG. 3 is a curve which illustrates the operation of a gas cell, and is useful in explaining the operation of this invention.

DESCRIPTION OF A PREFERRED EMBODIMENT Referring to FIG. I, there is shown a matrix of 16 gas cells 10. Each gas cell has an X-line which is connected to one part of the anode, and a Y-line which is connected to a second part of the anode in a manner which will be explained. All cathodes are tied together and return to a common supply point. Although a matrix of 16 gas cells is shown, any size matrix may be desired, and a typical display matrix might consist of 5 l0 gas cells.

FIG. 2 illustrates a typical gas cell 10 of the matrix shown in FIG. 1. The gas cell has a cathode 12, a first anode l4 and a second anode 16. The first anode l4 and the second anode l6 are electrically separated from each other by an insulating layer 18. In a practical embodiment, the main chamber for the gas cell might be formed as simply a round hole in a supporting body 19. A substrate 20 might have the second anode 16, the insulating layer 18 and the first anode 14 deposited successively in layers on it. In FIG. 2 equal electric field strengths are illustrated by lines 21. When a negative pulse is applied to the first anode 14, the electric field becomes distorted as illustrated by lines 21 in FIG. 2.

Referring to FIG. 3, there is illustrated a voltage-versus-current curve for a typical gas cell. The voltage lines on this curve imply a pure voltage source across the cell, but apply equally well to a cell with a series resistance if the voltage lines are drawn with a slope of UR where R,, is the series resistance. If the cell 10 were fired by simply applying a breakdown voltage across the cell, breakdown would occur at point 23 when the voltage rises above the knee 22 of the curve. However, if anode 14 is supplied with a negative voltage pulse while anode 16 is supplied with a positive voltage pulse, the knee 22 of the curve changes and breakdown occurs at point 24.

It is believed that this change in the voltage required to initiate a discharge is due to the distortion of the electric field 21 illustrated in FIG. 2. When the center anode 14 receives a negative voltage pulse the electric field lines bend toward the center anode. This distortion effectively moves the cathode closer to the anode in the center of the cell. The breakdown voltage of the cell depends on the product of the gas pressure in the cell times the electrode spacing in the cell. Generally, a larger product requires a higher voltage to cause an initial firing breakdown across the cell. Thus, the efiective closer spacing of the cathode causes the requisite breakdown voltage to decrease.

In an enlarged embodiment which was built and tested, the anode-to-cathode spacing was 0.4 inch and the gas cell was filled with argon at a pressure of 10 mm. Both of the anodes and the cathode were made of copper. The voltage which was normally maintained across the cell was 250 volts. If the first anode remained at the 250-volt level, the voltage on the second anode was required to be raised to 500 volts before breakdown of the cell occurred. When the first electrode was pulsed with a negative voltage of 200 volts bringing it to 50 volts relative to the cathode, discharge across the cell was initiated'when the second anode was raised to 340 volts.

The normal operating conditions of a gas cell will now be explained. As shown in FIG. 3, the initial firing voltage is nearly twice the operating voltage utilized to maintain a discharge in a gas cell. The normal operating condition for a cell would be at point 40. The operating voltage 34 is normally maintained across the cell even when the cell is not firing. In a typical prior art gas cell, the cell would be fired by applying a positive voltage pulse to the anode and a negative voltage pulse to the cathode, the magnitude of each of the pulses being approximately one-half the difference between the firing and operating voltages.

The application of a negative voltage pulse to anode l4 lowers the knee 22 of the curve from point 23 to point 24 so that less of a positive voltage pulse to anode I6 is required to initiate a discharge in the cell. The magnitude of a normal firing voltage is above the knee 22 of the curve. The effect of this higher magnitude is to shift the tum-on current to the positive- 1y sloped region 26 of the curve. When a discharge is initiated by raising the voltage above point 23, the tum-on current is at point 28. When discharge is initiated by raising the voltage above point 24, the tum-on current is at point 30. The difference in current between points 28 and 30 represents a significant reduction in tum-on current.

As mentioned previously, when the discharge is normally sustained across the cell, anodes 14 and 16 are maintained at the same voltage level to sustain the discharge. Each anode is tied to a separate voltage source, and either anode will sustain a discharge across the cell. Therefore, both anodes must simultaneously be lowered in voltage to lower the total voltage across the cell below the sustaining level 32 to turn the cell off. Thus, each anode ensures that the cell will maintain itself in a discharging condition, and both cells function in a logical AND-manner to turn the cell off. This AND-function allows the normal operating voltage level 34 to be lowered to voltage level 36 which is much closer to the extinguishing voltage level 32 of the cell. The lowering of the operating level represents a change in normal operating current from point 40 to point 42, and thus represents a significant reduction in operating current.

To ensure that the various command signals which are sent to a cell are performed properly, the firing voltage must be held at a given level above the operating voltage. As explained above, the logical AND-function performed by the two anodes allows the operating voltage to be lowered. This in turn allows the firing voltage to be lowered which is accomplished by reducing the gas pressure in the cell. The voltage required to initiate a discharge in a cell is dependent upon the gas pressure in the cell and may be varied by changing the gas pressure. This reduction in gas pressure results in a lower overall current in the cell, as the current varies as the square of the gas pressure. This overall reduction of current results in a lower power consumption by the cell. When the number of cells in a given matrix are considered, a significant reduction in operating power is achieved.

In summary, this invention allows a reduction in power consumption by a gas cell in three separate and distinct ways. The first is due to the lowering of the knee of the voltage-versuscurrent curve. The lowering of the knee allows the voltage required to fire a cell and the firing current to be reduced. The second reduction in power is due to the lowering of the operating current which is made possible by the AND-function of the two anodes. The third reduction in power results from the lowering of the operating voltage. This allows the firing voltage, which for command purposes must be maintained a certain magnitude above the operating voltage, to be reduced. The reduction of the firing voltage is accomplished by lowering the gas pressure in the cell, which results in an overall reduction of current flow in the cell since the current in the cell varies as the square of the gas pressure.

What is claimed is:

1. Apparatus for selectively initiating an electrical discharge in an electrical cell by applying initiating electrical signals to only one end of the cell, and comprising:

a. an electrical cell, including a first electrode means at a first end of the cell, a second electrode means at a second end of the cell opposite said first end of the cell, said second electrode means having a first electrode and a second electrode, and means for electrically isolating said first and second electrodes;

. means for sustaining an electrical discharge in said cell including means for applying a sustaining voltage between said first electrode means and at least one electrode of said second electrode means; and

c. means for initiating an electrical discharge in said cell by applying initiating electrical signals to only said second electrode means and including, means for applying an initiating voltage pulse of a first polarity to said first electrode of the second electrode means having a magnitude sufficient to distort the electric field in the cell as applied by said sustaining means and to lower the voltage required to initiate an electrical discharge in the cell, and means for applying an initiating voltage pulse of a second polarity opposite said first polarity to said second electrode of the second electrode means simultaneously with the application of said voltage pulse of a first polarity and having a magnitude sufficient to initiate an electrical discharge in said cell.

2. Apparatus as set forth in claim 1 wherein said first electrode means is the cathode of the cell, and said second electrode means is the anode of the cell.

3. Apparatus as set forth in claim 2 wherein said cathode has a plane surface at said first end of the cell, and said first and second electrodes of the anode have plane surfaces which are parallel to said plane surface of the cathode.

4. Apparatus as set forth in claim 1 wherein:

a. said electrical cell is located in a matrix of similar electrical cells;

b. said matrix includes a Y-electrical-connector for each row of the matrix for commonly connecting said first electrode of the second electrode means of each cell in a particular row of the matrix, and an X-electrical-connector for each column of the matrix for commonly connecting said second electrode of the second electrode means of each cell in a particular column of the matrix; and

c. said initiating means comprises means for initiating an electrical discharge in a particular electrical cell in the matrix, including means for applying an initiating voltage pulse of a first polarity to the Y-electrical-connector to which that electrical cell is connected, and means for applying an initiating voltage pulse of a second polarity opposite said first polarity to the X-electrical-connector to which that electrical cell is connected.

5. Apparatus as set forth in claim 4 wherein said cathode has a plane surface at said first end of the cell, and said first and second electrodes of the anode have plane surfaces which are parallel to said plane surface of the cathode. 

1. Apparatus for selectively initiating an electrical discharge in an electrical cell by applying initiating electrical signals to only one end of the cell, and comprising: a. an electrical cell, including a first electrode means at a first end of the cell, a second electrode means at a second end of the cell opposite said first end of the cell, said second electrode means having a first electrode and a second electrode, and means for electrically isolating said first and second electrodes; b. means for sustaining an electrical discharge in said cell including means for applying a sustaining voltage between said first electrode means and at least one electrode of said second electrode means; and c. means for initiating an electrical discharge in said cell by applying initiating electrical signals to only said second electrode means and including, means for applying an initiating voltage pulse of a first polarity to said first electrode of the second electrode means having a magnitude sufficient to distort the electric field in the cell as applied by said sustaining means and to lower the voltage required to initiate an electrical discharge in the cell, and means for applying an initiating voltage pulse of a second polarity opposite said first polarity to said second electrode of the second electrode means simultaneously with the application of said voltage pulse of a first polarity and having a magnitude sufficient to initiate an electrical discharge in said cell.
 2. Apparatus as set forth in claim 1 wherein said first electrode means is the cathode of the cell, and said second electrode means is the anode of the cell.
 3. Apparatus as set forth in claim 2 wherein said cathode has a plane surface at said first end of the cell, and said first and second electrodes of the anode have plane surfaces which are parallel to said plane surface of the cathode.
 4. Apparatus as set forth in claim 1 wherein: a. said electrical cell is located in a matrix of similar electrical cells; b. said matrix includes a Y-electrical-connector for each row of the matrix for commonly connecting said first electrode of the second electrode means of each cell in a particular row of the matrix, and an X-electrical-connector for each column of the matrix for commonly connecting said second electrode of the second electrode means of each cell in a particular column of the matrix; and c. said initiating means comprises means for initiating an electrical discharge in a particular electrical cell in the matrix, including means for applying an initiating voltage pulse of a first polarity to the Y-electrical-connector to which that electrical cell is connected, and means for applying an initiating voltage pulse of a second polarity opposite said first polarity to the X-electrical-connector to which that electrical cell is connected.
 5. Apparatus as set forth in claim 4 wherein said cathode has a plane surface at said first end of the cell, and said first and second electrodes of the anode have plane surfaces which are parallel to said plane surface of the cathode. 